Magnetic resonance is useful to detect the presence of a specific substance in a sample. For example, generally, radio frequency (RF) radiation at a particular frequency will induce a magnetic resonance signal in a specific substance, but not in other substances. Therefore, the induced magnetic resonance signal can be detected to thereby indicate the presence of the specific substance.
It is common to detect a magnetic resonance signal by placing a sample to be measured in or near a tuned, electronically resonant tank circuit. Then, the response of the tank circuit to the electromotive force produced by nuclear or electronic spins in the sample is measured. With Nuclear Magnetic Resonance (NMR), Nuclear Quadrupole Resonance (NQR), and low-frequency Electron Paramagnetic Resonance (EPR) the sample is placed in or near an inductor, commonly referred to as a coil, that detects AC magnetic fields. The inductance of the coil is tuned with a parallel and/or series capacitance to make the circuit electrically resonant at the measurement frequency. One or more additional reactive impedances (inductors or capacitors) are typically added to adjust the resistive impedance at resonance to a particular value which optimizes the detection sensitivity.
NQR detection systems for the detection of explosives and narcotics, and various NQR concepts, are disclosed, for example, in U.S. Pat. No. 5,233,300, “Detection Of Explosive And Narcotics By Low Power Large Sample Volume Nuclear Quadrupole Resonance (NQR)”; U.S. Pat. No. 5,365,171, “Removing The Effects Of Acoustic Ringing And Reducing Temperature Effects In The Detection Of Explosives By NQR”; U.S. Pat. No. 5,206,592, “Detection Of Explosives By Nuclear Quadrupole Resonance”; U.S. Pat. No. 5,608,321, “Method And Apparatus For Detecting Target Species Having Quadrupolar Nuclei By Stochastic Nuclear Quadrupole Resonance”; and U.S. Pat. No. 6,054,856, “Coil Which Is Immune To Environmental Noise”, all of which are incorporated herein by reference.
In most NQR, NMR and EPR applications, a common coil would typically be used as both a receiver coil and a transmitter coil, though this is not essential. In MRI separate coils would typically be used. The asymmetric gradiometer coils can be used for either or both of the transmitter and receiver coils.
FIG. 1 is a diagram illustrating an example of a conventional magnetic resonance apparatus. Referring now to FIG. 1, a transmitter 20 and a receiver 22 are connected to a probe 24 through a transmit/receive (TIR) switch 26. Probe 24 includes a coil 28, forming part of a resonant, tuned tank circuit 27 with various other inductors L and capacitors C as tuning elements. To detect the presence of a target substance, T/R switch 26 connects transmitter 20 to probe 24 while disconnecting receiver 22 from probe 24. Then, transmitter 20 generates a pulse and supplies the pulse to probe 24. As an example, in NQR, the pulse is formed from an RF signal having a frequency corresponding to the resonance signal of the target substance which is intended to be detected. Probe 24 receives the pulse, which causes coil 28 to store (RF) energy.
If a sample (not illustrated) is appropriately placed near or inside coil 28, the stored RF energy will cause a corresponding RF magnetic field to irradiate the sample. If the sample includes the target substance, the RF magnetic field will induce a magnetic resonance signal in the target substance. For example, if the apparatus operates under the principles of NMR (which includes Magnetic Resonance Imaging, MRI), then an appropriate NMR resonance signal will be induced. If the apparatus operates under the principles of NQR, then an appropriate NQR resonance signal will be induced.
After the sample is irradiated with the RF magnetic field, T/R switch 26 connects receiver 22 to probe 24 while disconnecting transmitter 20 from probe 24. Coil 28 then detects the resonance induced in the target substance, and probe 24 produces a corresponding output signal. The output signal of probe 24 is received and analyzed by receiver 22, to confirm the presence and/or measure the quantity of the target substance in the sample.
FIG. 1 is only one example of a magnetic resonance apparatus. For example, FIG. 1 illustrates T/R switch 26 to connect transmitter 20 and receiver 22 to the same probe 24. However, instead, a transmitter and receiver can each have a separate, dedicated probe together with a switch or gate for protecting the receiver while the transmitter is ON. Note that such an apparatus may also be used to measure material specific RF signals which arise from electromagnetic properties other than the magnetic resonance phenomenon.
FIG. 2 is a diagram illustrating a simple, conventional coil which can be used in a probe. Referring now to FIG. 2, a coil 29 typically forms a loop. Typically, a tuning capacitance C and a matching capacitance C′ are also provided. The coil shown has 1 turn, though additional turns are often used.
In magnetic resonance the signal-to-noise ratio (SNR) is determined, in part, by the noise contributions and the quality factor (Q) of the receiver coil. It is well-known that random thermal noise contributions typically arise from Johnson noise in the RF inspection coil and the first amplifier in the receiver. In the case where the probe cannot be electrically shielded, a further noise contribution arises from extraneous environmental noise.
It is also well-known in magnetic resonance that the Q of a receiver coil is determined not only by resistive loss in the windings of coil itself but also by loss in nearby electrically conducting materials that can dissipate energy from currents in the coil. As the SNR typically varies as Q1/2, such electrical loss in the surroundings leads to a reduction in SNR. For example, in MRI, the main source of electrical loss can come from the patient, and not the receiver coil windings, as the water in the body has an electrical conductivity comparable to sea water. In NQR land mine detection it is found that some soils also present significant electrical loading to the receiver coil, leading to a decreased coil Q and decreased SNR.
In many applications in MRI and also in land mine detection, a surface coil is used for the inspection. The larger the surface coil, the more the receiver Q is decreased by the presence of electrically conducting materials in the surroundings. Indeed, in MRI, a system designer conventionally chooses as the receiver coil the smallest surface coil that will “cover” the region of interest.
For a conventional simple circular coil (FIG. 2) of radius R the RF magnetic field falls off with distance so the coil is very inefficient at receiving NMR or NQR signals much beyond a distance R. Hence, for applications such as MRI or land mine detection, the distance to the inspection or interrogation region of interest determines the minimum size of the coil which is useful. For such a circular coil on the surface of a large conducting volume, we find that approximately 65% of the electrical loss from this volume arises from regions that are deeper than a distance R below the surface. Hence, most of the loss comes from a region beyond the actual region that can be imaged (in MRI) or inspected (in NQR landmine detection).
Although an RF coil used as a detector in magnetic resonance is not specifically designed to detect or receive radio signals, the detection coil does tend to pick up such unwanted interference. This radio interference from radio stations or other RF noise sources in the relevant frequency range can overwhelm the magnetic resonance signals of interest. One solution is to employ an external RF shield surrounding the coil and sample, however, this is impractical in many applications and can unduly increase the cost and size of the system. For the simple circular coil, the susceptibility for receiving interference from distance RF sources increases in proportion to the square of the coil radius, R.
Accordingly, it is desired to reduce the environmental noise pickup and reduce the electrical loss due to the proximity of a conducting medium while at the same time keeping adequate sensitivity to the desired signal. Approaches for achieving this may employ detector (or transmitter) coils that are designed to (i) strongly reject electromagnetic environmental noise from more distant sources, (ii) to couple strongly to the desired signals that arise from the desired inspection region very near the coil, and (iii) to couple more weakly to other nearby electrically conducting materials.
In most NQR, NMR and low-frequency EPR applications, a common coil would typically be used as both a receiver coil and a transmitter coil, though this is not essential. In MRI separate coils would typically be used. By the principle of reciprocity, all coil designs can be evaluated by considering them for use as a transmitter even if the coil is ultimately intended to be used as a receiver. That is, if when used as a transmitter a coil produces a large magnetic field at a specified location, that same coil can be expected to be a good receiver for magnetic resonance signals arising from material at that location.
One approach to reduce interference is ‘balancing’, in which the interference enters but is arranged to cancel itself based on some property of the detection coil and the external interference. A gradiometer is one example of this approach, as it can be designed to respond to any order of the spatial derivative of the electric or magnetic field.
A conventional linear or first order gradiometer is primarily sensitive to the first spatial derivative of the field, and is correspondingly insensitive to field components that do not vary in space. Since the spatial variations of an electromagnetic wave are characterized by its wavelength, this provides a means to cancel fields that have wavelengths much larger than the characteristic size of the gradiometer, thereby reducing noise pickup from distant sources. (For example, the free space wavelength of a 1 MHz signal is 300 meters whereas the coils will generally be less than 1 m across.). A simple conventional (linear) magnetic field gradiometer is formed of two loops which are spatially removed from each other and have currents flowing in opposite directions.
FIG. 3 is a diagram illustrating a conventional magnetic field gradiometer. Referring now to FIG. 3, a conventional magnetic field gradiometer has a conductor 30 which forms two loops 32 and 34 of identical radii but wound in the opposite sense. Each of these loops may have more than 1 turn of wire. The direction of the current changes in coils 32 and 34, so that magnetic fields generated by coils 32 and 34 are opposite each other.
For an application such as buried land mine detection, the inspection volume is located in one half space (underground) and the detection apparatus is limited to the other half space (above ground). The coil is used as part of a tuned circuit to detect the NQR signal which for landmine detection is a narrowband (˜1 kHz) signal in the range of 0.5-5 MHz. These NQR signals are very weak and RF interference is a major problem for such field work. While a conventional magnetic field gradiometer will reduce environmental magnetic field noise pick-up, the amount of reduction is often insufficient for many applications. In addition, there is also a reduction of the desired signal, compared to the simple circular coil, which is larger than desired. Here the use of an external RF shield, which must surround both the coil and the land mine, is not practical. It is therefore desirable to provide an improved gradiometer-type coil design having improved immunity to interference without extra shielding and at the same time providing an efficient detection region outside (i.e. near, but away from) the coil.